Motor vehicle fuel vapor emission standards have been in effect for some time, and are becoming increasingly stringent. Originally, evaporative emission control efforts were directed at preventing the loss to the atmosphere of fuel vapors emitted from the carburetor and the loss of fuel vapors generated in the fuel tank. The typical evaporative emission control system that arose from such efforts, which is commonly used in production today, includes a vapor storage canister charged with activated absorbent charcoal pellets. The carburetor and fuel tank are both connected to the canister through various arrangements of hoses and control valves. Current work is aimed as well at recovering the raw hydrocarbons lost to the atmosphere while the fuel tank is being filled. Several designs have been proposed and used to recover both the vapors generated from the fuel dispensing nozzle itself as well as the vapors displaced from the tank and up the filler pipe by the entrance of the fuel. Various arrangements of hoods and vent lines have been used to direct such fuel fill vapors away from the atmosphere and to the vapor storage canister. A lesser, though not inconsiderable vapor loss is the pressurized tank vapors that exit almost immediately to the atmosphere through the upper end of the fuel tank filler pipe when the gas cap is first removed. This loss has come to be called a "puff loss" due to the distinctive popping sound that occurs as the cap is removed, which is most noticeable on a hot day.
The vent system disclosed in the U.S. Pat. No. 4,572,394 to Tanahashi et al purports to deal with puff losses, but in a very impractical manner. As there disclosed, an additional canister would be provided surrounding the filler pipe itself. The removal of the gas cap would allow a bellows held within a large cone shaped upper end of the filler pipe to shift, purportedly quickly enough to simultaneously block off the upper end of the filler pipe and also direct puff loss vapors into the additional canister. The room taken by, and the added expense of, the extra canister, as well as the radical departure from conventional filler pipe design, make the system highly undesirable. U.S. Pat. No. 4,630,749 to Armstrong et al, assigned to the assignee of the present invention, controls fuel vapors with the storage canister of a conventional evaporative emission control system. The venting means there disclosed has a flapper door that engages an annular seal located below the cap, but upstream of a canister connected fitting that opens to the filler pipe. The fitting opening is normally closed by a spring loaded ball valve. Just after cap removal, the flapper door is still closed on the seal to prevent puff loss at that point. The flapper door does not open until the fuel nozzle is inserted to push it open, but, before the nozzle opens the flapper door, it passes through the annular seal, creating a new barrier to the puff loss. As the flapper door is pushed open, a projection on it in turn opens the ball valve to open a path for the pressurized fuel vapors, both the puff loss vapors and fill vapors that occur during the fuel fill operation. In this design, the control of puff losses depends on the engagement of the seal with a properly sized fuel nozzle. Absent that, an opening would be created to the atmosphere as soon as the flapper door was pushed open. While the flapper door would generally be pushed open only by such a properly sized nozzle, it is possible that it could be opened by a too small nozzle, or by some object other than a nozzle, as when the operator was adding something other than nozzle dispensed fuel to the fuel tank. Puff losses would not be controlled under those circumstances.